The present invention relates to an apparatus for optically measuring a deformation of an object.
A speckle method is known as one of various methods of optically measuring a deformation, including a movement, of an object. For example, Japanese Patent Application Unexamined Publication No. Sho. 57-187605 discloses an optical movement measuring apparatus utilizing the speckle method, in which a spatial light modulator such as a BSO is employed to perform a real-time measurement.
Further, there exist optical deformation measuring apparatuses employing other types of spatial light modulators such as a liquid crystal television, TN-LCLV and thermoplastic.
However, the BSO spatial light modulator necessarily makes the apparatus large because a write light source and a read light source should be separate ones.
In the case of using the liquid crystal television, no memory function is obtained, and therefore a frame memory is required to provide a double recording function. This causes a limitation on an interval between recording events.
Because of the lack of a memory function, the TN-LCLV apparatus utilizes the effective voltage effect of a liquid crystal. Therefore, it can only be applied to measurements of such deformation (movement) as simple harmonic motion.
Since the thermoplastic apparatus needs a longer recording time of several seconds, it cannot provide a real-time processing. Therefore, it can only be used for measurements in which movement speed or deformation speed of an object is slow.
In each of the above apparatuses, the sensitivity needs to be changed with the reflectivity of an object or a distance between the object and a recording device. However, the sensitivity of each apparatus cannot be changed arbitrarily.
Although it may be possible to change the measurement period instead of changing the sensitivity, this will cause limitations on the kinds of measurable deformations (movements) and on the measurement interval.
In the conventional apparatuses, which cannot change its sensitivity arbitrarily, it is required to insert an ND filter or the like in a path of a light beam to be incident on an object, to change the light intensity. This is a cumbersome operation.
On the other hand, when a laser light source is employed in the speckle photography, Young's fringe obtained by Fourier-transforming a laser speckle pattern is analyzed by an image analyzing apparatus. But, the image analyzing apparatus is not only expensive but also needs complicated computing software.
In the speckle photography, Young's fringe appears only in the case where the displacement of a speckle is larger than its average diameter. Therefore, the minimum measurable displacement is determined by the speckle diameter, and no measurement can be performed if the speckle displacement is too small.
Further, in the conventional speckle photography using a spatial light modulator, a speckle pattern is stored in the single spatial light modulator by double writing and a deformation is detected on a one-phenomenon basis. Therefore, a deformation between sets of deformations cannot be measured.